1. Field of the Invention
The present invention relates to a PLL (Phase-Locked Loop)-controlled oscillator for use in communication systems, and more particularly to a PLL-controlled oscillator to be incorporated in devices such as mobile phones or the like.
2. Description of the Related Art
PLL-controlled oscillators comprise a reference signal source and a voltage-controlled oscillator which is controlled by a PLL based on a reference signal from the reference signal source. The PLL-controlled oscillators are employed in various devices in cellular mobile communication systems in particular, because the oscillating frequency of the voltage-controlled oscillator changes in synchronism with or following a divided frequency from the reference signal source. Usually, the reference signal source comprises a temperature-compensated crystal oscillator, and the voltage-controlled oscillator comprises an LC oscillating circuit having a resonant circuit which is made up of an inductance (L) and a capacitance (C).
FIG. 1 shows a circuit arrangement of a conventional PLL-controlled oscillator by way of example. This PLL-controlled oscillator comprises temperature-compensated crystal oscillator (TCXO) 1 as a reference signal source, voltage-controlled oscillator (VCO) 2, first frequency divider (1st DIV) 11a for dividing a reference frequency from temperature-compensated crystal oscillator 1, second frequency divider (2nd DIV) 11b for dividing an output frequency from voltage-controlled oscillator 2, phase comparator (COMP) 12 for comparing the phases of an output frequency from first frequency divider 11a and an output frequency from second frequency divider 11b, and low-pass filter (LPF) 13 connected to an output terminal of phase comparator 12. Low-pass filter 13 generates a control voltage based on the phase difference that is detected by phase comparator 12, and applies the generated control voltage to voltage-controlled oscillator 2. With the circuit arrangement shown in FIG. 1, voltage-controlled oscillator 2 is controlled by a PLL. Specifically, the output frequency of voltage-controlled oscillator 2 follows the frequency of a signal which is produced when the reference frequency is divided by first frequency divider 11a. 
As shown in FIG. 2, temperature-compensated crystal oscillator 1 has a structure which comprises a casing 7 having a recess which accommodates therein quartz crystal unit 3 and IC (integrated circuit) chip 6. Casing 7 has a step in the recess, and crystal unit 3 consists of quartz crystal blank 3A having an end fixed to the step by electrically conductive adhesive 9. IC chip 6 is fixedly mounted on the bottom face of the recess. Cover 8 is bonded to casing 7 in covering relation to the recess, thus hermetically sealing crystal unit 3 and IC chip 6 in casing 7. Although not shown in FIG. 2, a connection electrode is disposed on the step and electrically connected to crystal blank 3A. The connection electrode is also electrically connected to IC chip 6 by a circuit pattern (not shown), thus electrically connecting IC chip 6 and crystal blank 3A (crystal unit 3) to each other.
As shown in FIG. 3, temperature-compensated crystal oscillator 1 has a circuit arrangement comprising crystal unit 3, oscillating circuit 4 connected to a terminal of crystal unit 3, voltage-variable capacitance element 10 such as a variable-capacitance diode connected to the other terminal of crystal unit 3 and ground, and temperature compensating mechanism 5 for measuring an ambient temperature, generating a temperature compensating voltage based on the ambient temperature, and applying the temperature compensating voltage to voltage-variable capacitance element 10. IC chip 6 includes an integrated assembly of oscillating circuit 4 and temperature compensating mechanism 5. A write terminal (not shown) is disposed on an outer side surface of casing 7 for writing temperature compensating data in temperature compensating mechanism 5.
Generally, crystal unit 3 comprises an AT-cut quartz crystal blank and has frequency vs. temperature characteristics represented by a cubic curve having an inflection point near the normal temperature of 25° C. Oscillating circuit 4 is constructed as a Colpitts oscillator using an inverter amplifier, and uses voltage-variable capacitance element 10 as an oscillating capacitor. Temperature compensating mechanism 5 has a sensor element such as a temperature-sensitive resistor for detecting an ambient temperature, and a voltage generating circuit for generating a temperature compensating voltage which varies according to a cubic function based on the detected temperature. The generated temperature compensating voltage is applied to voltage-variable capacitance element 10 to change a series equivalent capacitance (i.e., load capacitance) as seen across crystal unit 3, thereby compensating for a change in the resonant frequency of crystal unit 3 due to a change in the ambient temperature for keeping the oscillating frequency constant.
Voltage-controlled oscillator 2 comprises an LC oscillating circuit of the Colpitts type, for example, and includes a voltage-variable capacitance element inserted in the closed oscillation loop. The output frequency of voltage-controlled oscillator 2 varies in response to the control voltage that is applied to the voltage-variable capacitance element. Voltage-controlled oscillator 2 has a specific circuit arrangement which is basically equivalent to the circuit arrangement shown in FIG. 3 except that the temperature compensating mechanism is dispensed with and an LC resonating circuit is used in place of crystal unit 3.
The PLL-controlled oscillator described above is used in a transmission/reception system of a cellular phone, for example. One example of such a transmission/reception system of a cellular phone is illustrated in FIG. 4.
The transmission/reception system has temperature-compensated crystal oscillator (TCXO) 1 as a common reference signal source, transmission voltage-controlled oscillator (TXVCO) 2a which is PLL-controlled using the common reference signal source, and reception voltage-controlled oscillator (RXVCO) 2b which is PLL-controlled using the common reference signal source. For signal transmission, an output frequency in a 900 MHz band, for example, from transmission voltage-controlled oscillator 2a and modulated signal fa containing information such as audio information are mixed with each other by mixer 14a, and a generated high-frequency signal is transmitted via power amplifier (PA) 15 and radiated from antenna 16. For signal reception, a high-frequency signal received from antenna 16 and transmitted through low-noise amplifier (LNA) 17 and an output frequency from reception voltage-controlled oscillator 2b are mixed with each other by mixer 14b, producing demodulated signal fb. Information such as audio information is extracted from demodulated signal fb. As shown in FIG. 5, the above transmission/reception system physically comprises high-functionality IC 18 for generating a modulated signal and a demodulated signal, and discrete components 19 which serve as temperature-compensated crystal oscillator 1 and transmission and reception voltage-controlled oscillators 2a, 2b. High-function IC 18 and discrete components 19 being mounted on setting board (wiring board) 17.
Since temperature-compensated crystal oscillator 1 and voltage-controlled oscillator 2 are constructed separately from each other, the above PLL-controlled oscillator has low productivity and is manufactured at a high cost. It is difficult to reduce the physical size of voltage-controlled oscillator 2 because it is made up of capacitors and inductors constructed as chip elements and many discrete components including amplifiers and voltage-variable capacitance elements. The more the components used, the more complex circuit patterns for interconnecting the components becomes, tending to produce an electromagnetic coupling between the circuit patterns, which is likely to cause a trouble in the PLL-controlled oscillator.